Collecting samples to determine the potential for worker overexposure to contaminants is an integral component of industrial hygiene exposure assessments. So how do you know how many samples are actually needed? Where do you go for guidance?
While not the preferred method of determining how to conduct your exposure assessments, the reality is that budgets have a significant influence on how many samples you’ll collect, how much time you can spend with workers, how frequently you can revisit a work area, and the quality of laboratory service providers.
If you are under a tight financial constraint, you can work backwards from your final budget. You’ll need to account for analysis of the samples of concern, shipping, rental of any monitoring equipment or air sampling pumps, and if acting as a consultant to an outside client, the fees associated with your time to complete the site work and reporting activities.
Occupational Health and Safety Regulations
It would seem intuitive that occupational health and safety regulations would dictate a frequency of evaluating worker exposure, and to provide stipulations for the number of samples to be collected for a given size of workforce, however it’s often not the case. Regulatory wording typically provides a vague requirement that potential worker exposure must be evaluated but no further requirements are mandated. This is the point in which professional judgement can come into effect, both from the perspective of the individual conducting the assessment, and from regulatory officers judging the adequacy of the assessment relative to regulatory requirements. When in doubt, check the applicable regulations in your area. If no specific requirements are mandated, more is more in this case.
One of the most heavily referenced guidance documents on sampling strategies is the manual provided by the National Institute for Occupational Safety and Health (NIOSH) titled Occupational Exposure Sampling Strategy Manual. The document was generated in 1977 but is still an authoritative resource for industrial hygiene professionals to rely upon. While it provides reliable strategies for sample sizes, the manual also provides good basic information pertaining to the rationale for conducting assessments, relevant information that should be collected and recorded during an assessment, formulas for calculating potential airborne concentrations of contaminants for given evaporation rates and dilution rates, and strategies for selecting which workers will wear sampling equipment. For example, is it preferable to select the average employee or the worker in the highest risk role at a work site? Or should workers be selected randomly? The manual also provides guidance on the number of samples to be collected when you’re interested in comparing results to short term exposure limits (STELs) or ceiling exposure limits, particularly with respect to capturing a measurement from the top 20% or top 10% of exposures, at a given statistical confidence level.
If you’ll be conducting statistical analysis of your sample results, one of the best calculations in determining regulatory compliance is determination of the upper and lower confidence limits around the average exposure value. The manual walks you through the calculations in a straight-forward manner and provides support in how to compare the upper confidence limit (UCL) and lower confidence limit (LCL) to regulated exposure limits.
The key take-away is that the manual provides some black and white recommended numbers of samples to be collected for a given size of workforce, and that the assessor will quickly reach a point of diminishing returns, beyond which further sampling is unlikely to provide much added benefit. The hygienist can also use the resource as justification for assessment budgets and rationale for a sampling approach.
Another premier resource in protecting the health of workers is the l’Institut de recherche Robert-Sauvé en santé et en sécurité du travail (IRSST) based in Quebec, Canada. The IRSST has produced a sampling strategy guide, similar to the NIOSH manual, referred to as the Sampling Guide for Air Contaminants in the Workplace. In the Sampling Guide, a recommendation is made to separate the workforce into Similar Exposure Groups (SEGs), and refer to Table 1 of the guide for proposed number of samples to be collected for each SEG, in order to obtain at least one worker among the top 10% of most exposed workers, at a given confidence level.
Industry Association Guidelines
The American Industrial Hygiene Association (AIHA) is one of the best known associations in the field of industrial hygiene. While not freely available, their text A Strategy for Assessing and Managing Occupational Exposures is another of the foundational references for creating a sampling strategy. The text recommends “a starting minimum of 6 to 10 exposure measurements per SEG” and that more than 10 samples may not substantially add to the conclusions made during an assessment. If the exposure profile becomes increasingly close to the exposure limit, a greater number of samples may be required to be statistically confident that the exposure limit is not being exceeded. Of particular usefulness is the table provided in the text which outlines the number of samples required to be 95% confident the true mean exposure is less than the exposure limit. What differentiates these guidelines from others is that the variability of the exposures for a given exposure group will significantly influence the number of samples required for an average exposure level at a given fraction of the exposure limit. The table further outlines the number of samples required for given levels of variability when the average exposure value approaches the exposure limit.
The region in which the assessment is being performed may play into the number of samples to be collected as part of an exposure assessment. Care should be taken to verify the nature of the requirements in your area, such as determining the legal differences between guidelines provided by regulatory bodies, versus regulated and required numbers of samples. Occupational health and safety officers may also enforce guidelines for sampling strategies if specific regulations are not in place, with an expectation that deviances from the guidelines must be defensible and justifiable.
For example, the Ministry of Manpower in Singapore recommends that at least 3 to 5 samples for each job classification / group be collected, or 25% to 50% of those in the group for groups of 10 or more be sampled. Similarly, four to seven short-duration grab samples or spot measurements (using detector-tubes or other direct reading instruments) can be collected. Refer to the Guidelines on Sampling Strategy and Submission of Toxic Substances Monitoring document for further support.
For each sampling event, field blanks should be collected and submitted to the laboratory for quality control purposes. The blanks allow the industrial hygienist to determine if sample results can be attributed to the conditions in the work site, or if the sampling procedures may have unintentionally introduced contamination into the sampling media.
The IRSST recommends in the Sampling Guide that a number equivalent to 10% of the total number of samples should be collected and submitted for analysis. For example, if twenty air samples have been collected, the hygienist should submit two field blank samples along with the batch of samples.
The analytical procedure followed to collect the samples, such as a NIOSH or OSHA method, will also likely dictate a number of blank samples to be included, typically in the range of 2 to 10 samples, regardless of the total number of samples collected as part of an assessment.
Certain organizations, such as military forces, may dictate the number of field blanks to include. For example, the U.S. Army mandates that “a minimum of 1 field blank must be submitted for every 10 samples from the same sampling series, or any fraction thereof, even if there is only 1 sample in the set.”
Contaminant-Specific Sample Numbers
The information provided above pertains to the collection of air samples or measurements to evaluate the potential for worker (over)exposure to airborne contaminants. It should be noted that the sampling strategies may not be relevant for the collection of surface samples, or bulk samples of hazardous materials, such as asbestos-containing materials or lead-containing finishes. Many jurisdictions will provide specific recommendations for sampling materials suspected to contain asbestos. Similarly, many region-specific asbestos guidelines will stipulate the number of asbestos air samples to be collected during active asbestos removal activities. The occupational regulations in your area should be consulted to determine if contaminant-specific sampling requirements are mandated.
There is a significant number of variables which must be incorporated when determining the number of samples to be collected during an exposure assessment. The industrial hygienist should also incorporate experience and professional judgement when determining how to evaluate exposures to the workforce. Numerous resources are available to provide guidance on the number of samples to be collected.